The present invention relates to a hydraulic power unit for a vehicle, for example, capable of detecting an abnormal condition of a pressure of a gas filling in an accumulator of the hydraulic power unit.
In general, a vehicle such as an automobile is provided with a mechanically operatable speed changing apparatus which is electronically controlled. Such speed changing apparatus includes various kinds of actuators which are driven by a hydraulic power unit, and one typical example thereof is shown in FIG. 8.
Referring to FIG. 8, a hydraulic power unit is designated by a symbol PU which generally includes a motor 1, a gear pump 3, and a gas filling type accumulator 5, in which the gear pump 3 is driven by the motor 1 to pump up an oil from an oil tank 7 and the thus pumped-up oil is supplied to an actuator means 9 operatively connected to the hydraulic power unit PU. The actuator means 9 includes a clutch changing cylinder A, a shift changing cylinder B and a select changing cylinder C all of which are operated hydraulically and are adapted for a mechanically operatable speed changing apparatus. In the hydraulic power unit PU, a check valve 11 is incorporated in a line between the gear pump 3 and the accumulator 5 and a shift valve 13 is also incorporated in a line branched from the line connecting the gear pump 3 and the accumulator 5.
The hydraulic power unit PU of this conventional type is disclosed, for example, in the Japanese Utility Model Laid-open Publication No. 60-10901, in which the motor 1 is not always driven even during the operation of the actuator means 9. Namely, when a predetermined hydraulic, i.e. oil, pressure is accumulated in the accumulator 5, the operation of the motor 1 is once stopped at this moment and, during this motor operation stopping period, the actuator means b 9 is driven by the oil pressure accumulated in the accumulator 5, and on the contrary, when the oil pressure in the accumulatior 5 is lowered to or below the predetermined value, the motor 1 is again driven to thereby operate the accumulator.
Namely, as shown in FIGS. 8 and 9, a pressure detection switch 15 is incorporated to a line extending from an outlet port of the accumulator 5, and accordingly, the pressure detected by the pressure detection switch 15 is deemed as an accumulated pressure in the accumulator 5. When this accumulated pressure reaches a high level P.sub.H, for example, a contact 15a of the pressure detection switch 15 is opened to thereby deenergize a relay 19 through a control unit 17 and hence to open a contact 19a of the relay 19, thus stopping the operation of the motor 1. When the oil in the accumulator 5 is consumed gradually and the pressure therein has reached a low level P.sub.L, for example, the contact 15a of the pressure detection switch 15 is closed to thereby establish an electrical conduction to the relay 19 through the operation of the control unit 17 and hence to close the contact 19a of the relay 19, thus operating the motor 1. In FIG. 8, reference numeral 21 denotes a battery and 23 is a key switch.
During this operation, the actuator means 9 is operated, and as shown in FIG. 9, when the actuator means 9 is not operated, the operation of the motor 1 is also stopped and the pressure in the hydraulic line in the system can be maintained to the high pressure level P.sub.H.
In the above arrangement of the conventional hydraulic power unit of a vehicle, for example, since a pressure detected by a pressure detection switch 15 which is arranged at portion near to the outlet port of the accumulator 5 is deemed as an accumulated pressure in the accumulator 5, when an abnormal condition of the gas pressure in the accumulator occurs, a precise pressure control will not be expected. For example, when the gas pressure in the accumulator is abnormally lowered, the oil amount accumulated in the accumulator 5 is reduced over the pressure (i.e. pressure level P.sub.L) necessary for driving the acutuator means 9, and accordingly, the switching operation, that is on-off operation, of the motor 1 is increased in its frequency, thus providing a problem. Moreover, in the conventional arrangement, since the gas pressure in the accumulator 5 is not detected, it is difficult for a driver of a vehicle to be notified with such pressure abnormal. Such pressure abnormal will adversely affect a speed changing performance of a clutch when such hydraulic power unit is applied to the vehicle such as an automobile.